That’s increasingly possible, according to polls. As recently as late November, Mrs. Clinton led Senator Sanders by 25 points in the Hawkeye State, according to the RealClearPolitics average of major surveys. Now that same measure is essentially tied.

Some individual polls even have Sanders squeaking ahead in a caucus state where enthusiasm and momentum can make a real difference.

The problem for Clinton is that a loss in Iowa might well be followed by a second defeat in New Hampshire, where Sanders has long led. He’s a United States senator from neighboring Vermont, after all, and New Hampshire has many white liberal voters, who make up the core of Sanders’s support.

Two defeats right out of the gate? That might well strip the sheen of inevitability off Clinton’s campaign. No wonder she’s just released a digital ad supporting President Obama’s push for new gun control measures – an implicit rebuke to Sanders as soft on the issue. No wonder she’s dispatched Chelsea Clinton to attack Sanders on her behalf, saying that his health-care plans might dismantle Obamacare, and turn too much power over to state governors.

Clinton’s advantage is that she has vast support from party officials and elected Democrats. Long ago she ensured that she would enter the Democratic primaries as, in essence, the party’s choice. She crushes Sanders in endorsements, a measurement many political scientists consider key to winning nominations.

In addition, she leads Sanders by large margins among more moderate and conservative Democrats, and non-whites. These groups don’t happen to be numerous in Iowa and New Hampshire. But beginning in South Carolina, the third state on the electoral calendar, minority voters will play a major role.

Nate Silver at the data journalism site FiveThirtyEight did a lengthy analysis of this dynamic last year. He pointed out that it was entirely possible that Sanders could win Iowa and New Hampshire, then lose everywhere else. White liberals make up 54 of the New Hampshire Democratic electorate, for instance, but only 19 percent in South Carolina and 29 percent in Nevada, another early voting state.

However, the US presidential primary process is lengthy and interactive. In other words, the results in one state can influence the results in the next state, and so on down the line. Clinton’s challenge is this: She can’t afford to let Sanders build up enough momentum to overcome her institutional advantages.

Right now the former secretary of State seems both vulnerable and apparently invincible, as The Washington Post’s chief political correspondent Dan Balz points out. That paradox defines her campaign. It’s possible that her vulnerability will work to her advantage, spurring her and her aides to fight harder and hone their approach in advance of a surely tough general election. It’s also possible her invincibility will prove a mirage, and Sanders will upset her for the nomination. After all, he has proven his ability to raise lots of money. The Democratic race could stretch well into the spring.

“So much comes back to Iowa and the battle that will unfold over the next three weeks,” writes Mr. Balz.